Radial Nerve: Course, Branches and Clinical Anatomy

By Dr Richard Miller, MBChB FRCS · Reviewed 1 October 2026

The radial nerve is the largest branch of the brachial plexus, arising from the posterior cord and winding round the back of the humerus to reach the lateral elbow and the back of the forearm. It carries C5–T1 fibres, supplies every extensor of the arm and forearm, and gives sensation to the back of the arm, forearm and lateral hand.

Radial Nerve · key facts

Roots: C5–C8 and T1 via the posterior cord (T1 contribution inconstant)
Course: Triangular interval, radial groove of humerus, lateral intermuscular septum, between brachialis and brachioradialis
Motor supply: Triceps, anconeus, brachioradialis, all extensors of wrist, fingers and thumb, supinator, APL
Sensory supply: Posterior arm and forearm; lateral dorsum of hand and dorsal lateral three and a half digits short of the nail beds
Branches: Posterior cutaneous nerves of arm and forearm, inferior lateral cutaneous nerve of arm, superficial branch, deep branch (posterior interosseous)
Key relations: Profunda brachii artery in the radial groove; radial recurrent artery at the elbow; supinator
Injury: Humeral shaft fracture, Saturday night palsy, crutch palsy, posterior interosseous syndrome

Course

The radial nerve starts behind the axillary artery, spirals round the back of the humerus, and divides in front of the lateral epicondyle into a sensory and a motor branch.

In the axilla

It is the direct continuation of the posterior cord and lies behind the third part of the axillary artery, on subscapularis, teres major and latissimus dorsi. It leaves the axilla through the triangular interval, below the lower border of teres major, between the long head of triceps and the shaft of the humerus, with the profunda brachii artery.

In the arm

The nerve runs obliquely down and laterally in the radial groove on the back of the humerus, between the lateral and medial heads of triceps, in contact with bone for part of its length. In the lower third of the arm it pierces the lateral intermuscular septum and enters the anterior compartment, where it lies deep in the gap between brachialis medially and brachioradialis and extensor carpi radialis longus (ECRL) laterally.

At the elbow

In front of the lateral epicondyle the nerve divides into the superficial branch and the deep branch. The deep branch passes into supinator under the arcade of Frohse, the fibrous upper edge of its superficial layer, winds round the neck of the radius within the muscle and emerges on the back of the forearm as the posterior interosseous nerve (PIN).

In the forearm

The superficial branch runs down under cover of brachioradialis, lateral to the radial artery in the middle third, then turns backwards under the brachioradialis tendon a few centimetres above the wrist and crosses the anatomical snuffbox to the dorsum of the hand. The PIN runs between the superficial and deep extensors and ends on the back of the wrist joint.

Branches and supply

Because every posterior division of the plexus ends in the radial or axillary nerve, the radial nerve supplies all the extensors from the elbow down.

Level	Branches	Supplies
Axilla	Posterior cutaneous nerve of arm; muscular branches	Skin of the back of the arm; long and medial heads of triceps
Radial groove	Muscular branches; inferior lateral cutaneous nerve of arm; posterior cutaneous nerve of forearm	Lateral and medial heads of triceps, anconeus; skin of the lower lateral arm and back of the forearm
Above the lateral epicondyle	Muscular branches	Brachioradialis, ECRL, lateral part of brachialis
Elbow	Deep branch (PIN)	Extensor carpi radialis brevis (ECRB), supinator, then extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, extensor indicis
Forearm and hand	Superficial branch	Skin of the lateral dorsum of the hand and the dorsal lateral three and a half digits as far as the distal interphalangeal joints

The autonomous sensory zone of the radial nerve, where no other nerve overlaps, is the skin over the first dorsal web space between thumb and index.

Relations

The radial nerve is tied to the humerus in the radial groove and to supinator at the elbow, which is where it is injured.

Axilla: posterior to the axillary artery, the largest nerve there, lying on the muscles of the posterior wall.
Radial groove: with the profunda brachii artery and its venae comitantes, between the lateral and medial heads of triceps.
Lower arm: between brachialis and brachioradialis, with the radial collateral branch of profunda brachii and, lower down, the radial recurrent artery.
Forearm: the superficial branch lies lateral to the radial artery under brachioradialis; the PIN runs with the posterior interosseous artery.

Clinical relevance

The level of a radial nerve lesion is read from which extensors still work and whether the skin of the first web space is numb.

Level	Cause	Findings
Axilla	Crutch palsy, proximal humerus injury	Weak triceps and absent triceps reflex, plus everything below
Radial groove	Humeral shaft fracture, Saturday night palsy (arm draped over a chair back)	Wrist drop and finger drop, weak brachioradialis, first web space numbness; triceps largely spared
PIN	Radial head fracture or dislocation, arcade of Frohse compression, ganglion, surgery at the radial neck	Finger and thumb drop without wrist drop; wrist extends in radial deviation because ECRL is spared and ECU is weak; no sensory loss
Superficial branch	Tight watch strap or handcuffs, de Quervain release	Numbness or pain over the dorsoradial hand only (Wartenberg's syndrome)

Triceps is usually spared in a humeral shaft fracture because most of its branches arise before the nerve enters the radial groove. A spiral fracture of the distal third of the shaft (the Holstein–Lewis pattern) traps the nerve where it pierces the lateral intermuscular septum. Most closed injuries are neuropraxias that recover, which is why many surgeons observe before exploring.

How it is examined

On a cadaveric spotter, in an OSPE and in MRCS Part B anatomy.

In an axilla prosection the thick nerve lying directly behind the axillary artery is the radial; lift the artery forward to see it. The axillary nerve beside it disappears backwards at the lower border of subscapularis.
At the elbow, a nerve pinned in the gap between brachialis and brachioradialis is the radial nerve; follow it down to show the deep branch entering supinator and the superficial branch continuing under brachioradialis.
A pin on the nerve disappearing into supinator is the deep branch of the radial nerve, which becomes the posterior interosseous nerve; do not call it the superficial radial nerve.
The classic viva question is why triceps survives a mid-shaft humeral fracture, followed by how wrist drop differs from a PIN palsy (radial deviation, no sensory loss).
Know the extensor compartments at the wrist: every tendon in them is supplied by the radial nerve, and all but ECRL by the PIN or its deep branch.

Key points

The radial nerve is the continuation of the posterior cord and the largest branch of the plexus.
It leaves the axilla through the triangular interval and runs in the radial groove with profunda brachii.
It supplies triceps, anconeus, brachioradialis and every extensor of the wrist, fingers and thumb.
It divides in front of the lateral epicondyle into a superficial sensory branch and the deep (PIN) branch.
A radial groove lesion causes wrist drop with first web space numbness and usually spares triceps.
A PIN palsy causes finger drop without wrist drop and without sensory loss.

Common questions

What does the radial nerve supply?

The radial nerve supplies triceps and anconeus in the arm, brachioradialis and the lateral part of brachialis, and every muscle in the extensor compartment of the forearm: the wrist extensors, supinator, the finger extensors and the long abductor and extensors of the thumb. Its skin territory is the back of the arm and forearm, the lateral dorsum of the hand and the backs of the lateral three and a half digits short of the nail beds.

What causes wrist drop?

Wrist drop is the inability to extend the wrist, usually from injury to the radial nerve in the radial groove of the humerus. Typical causes are a humeral shaft fracture, compression when the arm hangs over a chair back during deep sleep, and prolonged pressure from crutches in the axilla. The hand hangs flexed, the fingers cannot be straightened at the knuckles, and the first dorsal web space is numb.

What is the difference between the radial nerve and the posterior interosseous nerve?

The posterior interosseous nerve is the continuation of the deep branch of the radial nerve after it passes through supinator. It has no cutaneous branch and supplies the forearm extensors except brachioradialis and ECRL, which the radial nerve reaches above the elbow. So a posterior interosseous palsy drops the fingers and thumb but not the wrist, and causes no numbness.

Why is triceps spared in a humeral shaft fracture?

Triceps is usually spared because most of its nerve branches leave the radial nerve in the axilla and upper arm, before the nerve enters the radial groove where a mid-shaft fracture injures it. The patient therefore keeps elbow extension and often the triceps reflex, while losing wrist and finger extension and sensation over the first dorsal web space.

References

Gray's Anatomy: The Anatomical Basis of Clinical Practice. Standring S (ed). Elsevier, 2020.
Moore's Clinically Oriented Anatomy. Moore KL, Dalley AF, Agur AMR. Wolters Kluwer, 2022.
Last's Anatomy: Regional and Applied. Elsevier.